That is a good point on maximizing space on mobile. But it actually shows a category dropdown on mobile, while the fullscreen view on desktop shows nothing at all.
Maybe on mobile a single toggle can switch between Writing and Settings: with an overview of all possible options including category, tags, enabling voting, whisper, visibility, timers etc. That would free up even more space, simplify, and add more features all in one go.
Fullscreen desktop mode should just show all of them options i guess.